How important are the eyes in reading and learning disabilities? This specific question is asked often by parents in pediatric ophthalmology practices. It is certainly true that the development of reading skills forms the basis for a strong education. It has been said that children attend school for the first few years to learn to read; after that, they read to learn. Concern about poor school performance associated with poor reading can overwhelm parents.
Physiology of Reading
Reading involves many highly sophisticated steps and the coordination of several central nervous system functions. It requires the acquisition of mapping between phonology and orthography, between speech sounds and letter symbols.
Reading begins with the decoding of serial visual information. Each letter must be resolved and then identified. Following this, groups of letters must be decoded to form phonemes, or sound groups, allowing proper identification of words. When reading, the attention of the reader and the eyes must be focused on the text of interest. The visual system must process and integrate multiple stimuli; thus, it is important to ensure its integrity.
Logically, one of the most basic and important recommendations in the evaluation of a child with reading difficulties is the assessment of visual acuity and the refractive status of the eyes required to achieve best corrected visual acuity. Although 20/20 vision is not essential, inadequate refractive correction may interfere with other visual processes required for reading, such as accommodation, binocularity, and ocular motility.
The task of reading requires the eyes to focus on a near target, inducing the near triad of accommodation, convergence, and miosis. Reading also requires stereotypical eye movements to visually sample, inspect, and process the text on a page, making the acquisition of information possible in a way that allows meaning to be attained. Eye movements show a clearly defined strategy during reading. The eyes are mostly constrained to a linear progression of saccades (with few regressions) to the right (in most languages) across the page, which allows the words to be read in an interpretable order.
Regressions happen when the reader reprocesses a previously read word, and they are more prevalent when reading challenging text and in children learning to read. Visual information is taken during periods of fixation between saccades. As a child's reading skills improve, the fixation time decreases and saccadic distance increases. When reading aloud, fixation periods are longer than during silent reading. Other factors that influence the duration of fixations include frequency of a word in daily activities and predictability of a word from the preceding context and familiarity with a word. Interestingly, word skipping is common with skilled readers. Short words, words that are predictable from the precedent context, and high-frequency words are more likely to be skipped by skilled readers without any loss of meaning.
After the eyes fixate on text, retinal processing transforms the letters and words into a neural code that is sent to the lateral geniculate nucleus and ultimately to the visual cortex. Cortical processing allows printed text to be properly interpreted through object recognition, spatial location, memory encoding, memory retrieval, and finally comprehension.1-3
Recent research supports the fact that vision problems are not the cause of learning disorders. Importantly, however, visual system disorders can make it very difficult for some children to effectively respond to classroom learning that requires sustained visual effort at reading distances for extended periods of time. Therefore, requests for ophthalmic evaluation from schools, primary care physicians, and therapists are common.
Most visual system disorders do not interfere significantly with reading. Convergence insufficiency, however, is one of the few ocular anomalies that can interfere with the physical act of reading. Convergence insufficiency may lead to symptoms of asthenopia, blurred vision at near distance, and the sensation that letters and words run together, especially with prolonged reading. Occasionally children report horizontal diplopia, nausea, and/or headaches at near distance. These symptoms may become more evident in the school environment because of increased demand for near work, and can affect a child's academic performance. Importantly, not all children with convergence insufficiency report asthenopia or even complain of symptoms.2
Relationship between Visual Processing and Reading Disabilities
With the development of functional MRI, it has become possible to study the areas of the brain activated during reading. There are 3 major parallel channels that carry information from the retina to the striate cortex via the dorsal lateral geniculate nucleus (LGN):
- Magnocellular
- Parvocellular
- Koniocellular
There is controversial evidence that an abnormality in the magnocellular visual pathway may play a role in reading disabilities. The magnocellular theory proposes that dyslexia is caused by impairment in the visual system, specifically from dysfunction of magnocells in the lateral geniculate nucleus, producing a deficit in the processing of visual information at low luminance, low spatial frequency, and high temporal frequency. Evidence for the magnocellular pathway deficit in dyslexia includes:
- Abnormally small magnocellular layer LGN cells
- Reduced or delayed electrophysiological responses
- fMRI responses to stimuli processed mainly by the magnocellular pathway5, 6
However, a recent study showed no significant relationship between visual measures and reading ability.7 Researchers argued that this could be because visual deficits account for some cases of reading disabilities independently of the phonological deficit. Other studies have also suggested that only a restricted subset of dyslexics have sensorimotor disorders. Considering this, there seems to be little value in the training of binocular control in children who have no visual impairment. It is believed now that there is an association between phonological dyslexia and sensorimotor syndrome (including auditory, visual, and motor disorders), which however, does not directly explain the reading disability.
In summary, although the visual system cannot be blamed for reading disabilities, its integrity needs to be assured to rule out any abnormality that could interfere with the reading process. The study of the brain is intriguing, and continued investigation is warranted. Understanding how the brain works in normal and reading-disabled individuals will guide the development of better treatments directed toward specific disabilities.
Recommended Work-up
A detailed history should be taken from the patient and parents/caregivers/guardians. Proper questions will help evaluate if the complaint is related to any ocular abnormality and guide you throughout the exam.
In the authors' center, all patients undergo a comprehensive ocular examination independently of the complaint and include assessments of:
- Best-corrected visual acuity
- Refractive status of the eyes, under cycloplegia
- Stereoacuity
- Head posture (and assessment of ocular cyclotorsion with specific tests, if suspected)
- Presence of nystagmus - careful attention to torsional nystagmus and fusion maldevelopment nystagmus (formerly known as manifest-latent nystagmus)
- Ocular alignment (phoria, intermittent tropia, manifest tropia)
- Ocular motility (ductions and versions)
- Smooth pursuits and saccades, to evaluate presence of dysmetria, commonly seen in neurologic pathologies as cerebellar ataxia
- Near point of convergence, checking breakout and recovery points
- Fusional amplitudes (convergence and divergence - vertical amplitude is tested in specific cases), using near target and prism bar
- Amplitude of accommodation, using near target and accommodative rule
- General ocular health, including slit lamp and dilated fundus examination